How long will a generator run on propane?

Propane is popular for backup power because it stores well and burns cleanly, but runtime numbers are not always obvious.

This guide looks at rough runtimes for common propane cylinder sizes and shows a simple way to estimate runtime for your specific generator.

Typical propane runtimes by tank size

Propane generators burn fuel at different rates depending on size and load, but some rough ballpark numbers can help with planning. These assume a typical portable generator running at about 50% load:

20 lb grill cylinder (about 4.7 gallons): roughly 8–12 hours.
30 lb cylinder (about 7 gallons): roughly 12–18 hours.
40 lb cylinder (about 9.4 gallons): roughly 18–24 hours.
Larger stationary tank (100+ gallons): many days of intermittent use, depending on draw.

These are only broad estimates. Actual runtime depends on generator efficiency, actual load, and how often the generator is cycling heavier loads like A/C or well pumps.

Propane vs gasoline runtime

Propane often gives slightly shorter runtime per unit of energy compared with gasoline, but it stores better and is cleaner burning. Many dual-fuel generators publish separate runtime specs for gasoline and propane at 25% and 50% load on various tank sizes.

If you have manufacturer specs, always use those numbers first. If not, you can estimate runtime with a simple formula using fuel consumption rate.

A simple way to estimate propane runtime

If your generator manual lists fuel consumption in gallons per hour (GPH) or pounds per hour, you can estimate runtime like this:

Runtime (hours) = Fuel amount (gallons) ÷ Fuel consumption (gallons per hour)

For example, if a generator burns 0.6 gallons of propane per hour at the load you're using and you have 9.4 gallons available (a 40 lb cylinder), then:

9.4 ÷ 0.6 ≈ 15.6 hours of runtime

If your manual lists fuel consumption in pounds per hour, convert pounds of propane to gallons (around 4.24 lb per gallon) or simply plan in terms of pounds instead of gallons.

Planning fuel for outages

When planning propane for longer outages, consider:

How many hours per day you truly need the generator running.
Whether you can cycle loads (like A/C) instead of running everything constantly.
How easy it is to get cylinders refilled during storms or extended events.

Tip: you can use the runtime, fuel cost, and size planners on the GeneratorCalc home page to turn these ideas into rough numbers for your specific generator and situation.